A strange signal from space

This week on Unexplainable or Not, the newest member of our team, Julia Longoria, tries to figure out which of three scientific mysteries about the sea, the land, and the sky has actually been solved. Guest: Admir Masic, associate professor of civil and environmental engineering at MIT. For show transcripts, go to vox.com/unxtranscripts For more, go to vox.com/unexplainable And please email us! [email protected] We read every email. Support Unexplainable by becoming a Vox Member today: vox.com/members Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Speaker 00

  Support for the show comes from TikTok. If you listen to this show, you probably enjoy discovering new things. It is built into the DNA of the show that curiosity is its own reward and that there's always a deeper dive to do. And one place to explore some of the topics we've covered here is TikTok. Whether it's how to forecast volcanic eruptions, strange phenomena that the James Webb Space Telescope picked up near the beginning of time, or a literal deep dive on the ocean's deepest reefs, TikTok has a wealth of fascinating videos just waiting to be discovered. Your next discovery is waiting on TikTok. Learn more at tiktok.com/discovermore.

• 0:49Speaker 01

  Support for the show comes from Engine. Running a small business means every dollar has to work hard. But if your team is still booking travel the old way, it's costing you more than you think. Engine is the fastest growing travel and spend platform in the country, built specifically for businesses like yours. Book a trip in as little as two and a half minutes, earn up to 10% back on hotels. And in 2025, Engine customers save more than $300 million on travel with zero booking fees, no contracts, and no BS. More than 1,000 businesses join Engine every month. Join them and get $500 when your business signs up and starts traveling at engine.com/box.

• 1:35Noam Hassenfeld?

  This is Unexplainable or Not, the game show where we finally get some answers. This week, our guest is Julia Longoria. She's been working on a new unexplainable series all about AI that's going to be coming out next week. And then, after that, she's going to be joining the Unexplainable team permanently. Woo! Welcome, Julia. Thank you. I'm so happy.

• 1:59Speaker 03

  Happy to be here with you all.

• 2:01Noam Hassenfeld?

  We are extremely excited as well. How are you feeling about making your debut on solving some high-pressure, potentially unsolved mysteries?

• 2:10Speaker 03

  I am shaking in my boots. I'm terrified.

• 2:14Noam Hassenfeld?

  We love to welcome people when they're as nervous as possible. So here's what we're doing. Unexplainable or not, is a game show where you have to guess what we know and what we don't. You're going to hear three scientific mysteries, and you're going to hear them from me, from our supervising producer, Meredith Hodnap.

• 2:31Speaker 01

  Hey there.

• 2:32Noam Hassenfeld?

  And from our senior reporter-producer, Byrd Pinkerton.

• 2:36Speaker 00

  Hello.

• 2:36Noam Hassenfeld?

  Two of these mysteries are still unexplainable, but one of them has recently been figured out. After you hear all the mysteries, you're going to get a chance to guess which one you think scientists have actually explained. And this week, in honor of you, we're doing a whole Julia-themed show.

• 2:55Speaker 03

  Oh, my goodness.

• 2:55Noam Hassenfeld?

  You've worked on some of our favorite shows out there, from More Perfect to Rabbit Hole to Radiolab. It's honestly kind of crazy how many amazing shows you've worked on.

• 3:06Speaker 03

  Guys, thanks.

• 3:07Noam Hassenfeld?

  And each of us is going to tell you a mystery related to one of those shows.

• 3:15Speaker 00

  Ish.

• 3:16Speaker 03

  I am so intrigued by what that means. Like, I just don't know. I'm really excited to hear what the mysteries are.

• 3:23Noam Hassenfeld?

  Let's just say, don't get your hopes up on the connections.

• 3:27Noam Hassenfeld?

  But first up, you're going to hear a mystery from Bird.

• 3:30Speaker 00

  Hello, Julia. Welcome to the show.

• 3:33Speaker 03

  Hi, Bird.

• 3:34Speaker 00

  So I love to overachieve, so I have actually hit two of your past jobs.

• 3:40Speaker 03

  Oh, my god.

• 3:40Speaker 00

  In that my mystery is set in the Atlantic, also the name of a publication you worked for.

• 3:48Speaker 03

  Okay, now I'm getting how tangential we are. I love it. Okay.

• 3:52Speaker 00

  And it's also about holes, like your past show, Rabbit Hole. So these are extremely real connections, but mostly this is just an excuse for me to do my favorite thing, which is to acknowledge that the ocean is full of Mysteries that we are trying to understand. And so today, I want to focus on an ocean riddle that was turned up by NOAA. That's the National Oceanic and Atmospheric Administration.

• 4:23Speaker 03

  Two As.

• 4:23Speaker 00

  Yes, the NOAA.

• 4:25Speaker 03

  Ah.

• 4:26Speaker 00

  So in 2022, a NOAA ship was sent out to explore a bunch of spots in the Atlantic, especially along the mid-Atlantic ridge. And in one of their dives, they spotted these weird holes on the ocean floor. So this is like 2,500 meters down.

• 4:45Speaker 03

  How far down?

• 4:46Speaker 00

  So this was 2,500 meters down, like 1.5 miles about.

• 4:50Speaker 03

  Okay.

• 4:51Speaker 00

  And like, they look almost rectangular. They're a little over a centimeter wide, a few centimeters long. And they appear in a straight line, like a line of holes, one after another, almost like a row of stitches.

• 5:06Speaker 03

  Yeah, it really looks like a kid playing in the sand, like doing like-

• 5:09Noam Hassenfeld?

  Really deep underwater.

• 5:12Speaker 01

  A mile and a half under the Atlantic.

• 5:15Speaker 00

  They don't look unintentional, essentially.

• 5:19Speaker 03

  Yeah, they look very intentional, yeah.

• 5:21Speaker 00

  And it's not the first time that holes like these have been spotted. There were others seen back in 2004, also in the Atlantic. And the scientists who wrote a paper describing those holes noted that they look kind of similar to holes that they'd found in fossilized rock, which suggests, right, that they're probably not man-made, if they're also in the fossil record. But then, how are they made? I reached out to NOAA, and they sent me a statement that said essentially, it could be some kind of process happening. So maybe there's some kind of tectonic activity in the area, releasing gas bubbles that come up to the surface and make these holes in some way.

• 6:04Speaker 03

  I feel like gas bubbles wouldn't make that, though.

• 6:07Speaker 00

  Yeah, it's like, they're so rectangular looking that I was kind of like skeptical as well. But they're also considering organisms that might do this. Like, maybe something's moving along the surface and- Making these or they've considered burrowing organisms because they're little mounds of sediments by the holes. So to kind of check all this out, one of the things that they did was to collect some environmental DNA from the site. So that's basically like DNA fragments that are just floating around. And the idea there is to look at those DNA fragments and compare them, see if they match up with anything in our like vast catalog of basically like everything that we have seen before, essentially.

• 6:51Speaker 00

  And in the statement that they sent, this lab director named Dr. Alan Collins said, "The little DNA fragments they gathered near these holes did not match anything in their data sets." There's just so much down at the bottom of the ocean that we have not catalogued yet, right? That we can't always use our catalogs to figure out what's going on. And so that sort of leaves us with this mystery of what are these holes? Where did they come from? It feels a little absurd to think that you can encounter something like this 20 years ago and then encounter it again in 2022 and still not have clear answers.

• 7:34Speaker 00

  But maybe it feels so absurd because we do have clear answers. And this is not a testament to how very little we know about the ocean floor. And it is instead a mystery we have solved.

• 7:48Noam Hassenfeld?

  So Julia, what do you think? Do you think we might have solved this one? Or do you think this is an unsolved mystery?

• 7:55Speaker 03

  I feel like we would have solved it. That is what my gut is saying.

• 7:59Noam Hassenfeld?

  They are just holes.

• 8:00Speaker 03

  They are just holes.

• 8:01Speaker 01

  How hard could it be?

• 8:02Speaker 03

  Yeah.

• 8:03Noam Hassenfeld?

  All right. Just remember the gut feeling when you start getting more mysteries later on.

• 8:09Speaker 03

  Yeah.

• 8:10Noam Hassenfeld?

  Next up, Meredith has her own mystery for you.

• 8:13Speaker 01

  All right. Julia, hello.

• 8:15Speaker 03

  Meredith, hello.

• 8:18Speaker 01

  My mystery today is all about concrete, which not a direct tie to your resume, but we'll, we'll get there. It'll all come around.

• 8:28Speaker 03

  I'm on the edge of my seat.

• 8:29Speaker 01

  I know. Fascinating stuff. So, concrete is the most used building material in the world. We're pumping out billions and billions of tons of it every year for our buildings, our roads, our dams. Like, concrete is basically the bedrock of modern life. But there's a concrete mystery that stretches back thousands of years to the Roman Empire. Just like us, you know, Romans, they used their concrete to build their roads, their aqueducts, their cities, even their marketplaces. You got some experience with marketplace, right, Julia?

• 9:06Noam Hassenfeld?

  Hey!

• 9:08Speaker 03

  Nice.

• 9:09Speaker 01

  And their temples, like the Pantheon, built 2,000 years ago. This huge unreinforced concrete dome that you can still see today. And that is exactly the point. That concrete is still there in the middle of Rome. And it's not just in beautifully preserved historical buildings. You can see this ancient concrete in seawalls, like submerged in saltwater, getting pounded by waves, the harshest conditions. And these walls are still standing thousands of years later. And we don't know why. Some scientists think it has to do with the chemistry of the Romans, like raw materials.

• 9:52Speaker 01

  There's this very special volcanic ash that came from like one very specific seaside town that Romans shipped across the empire to make their concrete. Other researchers think that the durability of this Roman concrete came down to how they mixed all of their, like, concrete ingredients together. Either way, it's extremely hard to figure out why this concrete lasts so long. Because all we have are like the surviving remains, like chemically altered by millennia of wear and tear and, you know, being exposed to the elements. It's also impossible to tell what was like purposeful engineering and what was just kind of like a lucky accident in material science.

• 10:39Speaker 01

  But one thing that we do know is that our modern concrete doesn't stand up nearly as well.

• 10:46Speaker 03

  That was gonna be my question, like,

• 10:47Speaker 01

  Yeah.

• 10:48Speaker 03

  What's the shelf life on concrete?

• 10:50Speaker 01

  Well, so I asked a professor of engineering at MIT, Admire Mashich, and he told me that modern concrete has a shelf life of 50 to 100 years, maybe 150 years, like if you're stretching it.

• 11:05Speaker 03

  Wait, we outlive concrete?

• 11:06Speaker 01

  Yeah.

• 11:07Noam Hassenfeld?

  Or some concrete?

• 11:09Speaker 03

  Some concrete.

• 11:09Noam Hassenfeld?

  Depending how long we live, right?

• 11:11Speaker 00

  But this is like a real issue with like, if you look at the Frank Lloyd Wright building that was supposed to be like a waterfall building or something and they want to preserve it, but it's like falling apart because so much of it is concrete, like,

• 11:23Noam Hassenfeld?

  Oh, interesting.

• 11:24Speaker 00

  It's very tragic.

• 11:26Speaker 01

  Totally. And it's not just architectural marvels we're talking about here. Our modern concrete infrastructure is continually crumbling. It needs to be replaced and repaired, even as we're going on to build just like more and more and more out of our concrete. And this has a huge environmental toll. The production of concrete is responsible for like billions of tons of carbon released into the air, accounting for as much as like eight or 9% of just all greenhouse gas emissions, just from concrete.

• 11:59Speaker 00

  Wow. Wait, why is it so like, it's just rock? Why is it so, or it's, it's just little?

• 12:08Noam Hassenfeld?

  Is concrete rock?

• 12:10Speaker 00

  What is concrete? But why does it emit so much CO2?

• 12:14Speaker 01

  So the process of taking limestone and making it become like this liquid rock of cement that then gets mixed into concrete, you're basically like heating it up so much that you're stripping out carbon dioxide. It's like pound for pound.

• 12:31Speaker 00

  Got it.

• 12:32Speaker 01

  Every pound of cement made releases just about a pound of CO2 into the air.

• 12:38Speaker 00

  That's so inefficient.

• 12:41Speaker 01

  Yeah, so figuring out the secret of Roman concrete, what makes it so durable? What makes it last so long? Could be the key of making our own long lasting concrete, you know, that doesn't need to keep being replaced and dramatically cut the carbon cost of all of our infrastructure.

• 13:00Speaker 03

  You know, you're really blowing my mind because I'm like, how I didn't quite realize we were building our societies on like such shaky foundations.

• 13:08Speaker 01

  Yeah.

• 13:09Noam Hassenfeld?

  Shaky and like, super environmentally harmful.

• 13:11Speaker 03

  Yeah, yeah, like, why are we doing that?

• 13:13Speaker 01

  Yeah.

• 13:14Speaker 03

  Isn't there better stuff to use? I guess, I don't know.

• 13:17Speaker 01

  Yeah, maybe they figured it out.

• 13:19Noam Hassenfeld?

  So what do you think? Do you think we know why Roman concrete was so durable? Or do you think that's still an unsolved mystery?

• 13:28Speaker 03

  I'm leaning toward it's an unsolved mystery, but I guess I just found out that we are building on shaky foundations, so maybe we aren't. But I'm like, very, highly impressionable.

• 13:39Noam Hassenfeld?

  Well, just, just hold off till the third mystery.

• 13:42Speaker 03

  Yep.

• 13:43Noam Hassenfeld?

  And this one is inspired by your time at Radiolab.

• 13:47Speaker 03

  Nice.

• 13:48Noam Hassenfeld?

  Specifically, the radio part. I want to tell you about something called the WOW Signal. So, in Ohio, in the 70s, there was this telescope called Big Ear. It wasn't a normal telescope with lenses and mirrors. It was a radio telescope, which means it was essentially listening for radio waves. And it would kind of output the data that it got on these spreadsheets. And an astronomer was going over this data, and he noticed this really weird signal. There's no recording of it, but there is a printout of the data, which I can show you here.

• 14:29Speaker 03

  Wow.

• 14:31Noam Hassenfeld?

  Wow.

• 14:32Speaker 03

  Someone just wrote, "Wow!" exclamation point. I assume that's not the signal, but...

• 14:37Noam Hassenfeld?

  No, no, that's, that's where the name comes from. The guy basically circled the signal, which was just this huge spike in energy. And he wrote, "Wow." So, WOW Signal. But the next time that patch of sky where the signal came from was analyzed, it was gone. And the signal was never recorded again.

• 14:56Speaker 00

  Is there a chance that, like, a bird fell on something and no one noticed?

• 15:03Noam Hassenfeld?

  Well, the thing is, it wasn't just that this signal never happened again. The signal was kind of inherently strange because it was just in one narrow frequency. So essentially like one note on the keyboard, which means that it probably wasn't a natural signal from space because if it was any kind of supernova or a pulsar or just something that happens all the time in space, that would have sent all kinds of frequencies. And the frequency it was sent at happens to be the same frequency that hydrogen emits radio waves at, which got people ultra excited because this is the

• 15:45Noam Hassenfeld?

  frequency that a lot of astronomers and scientists think that aliens would probably choose to communicate with us because they would probably try to communicate on the most universal frequency, you know, most common element in the universe, hydrogen.

• 16:02Speaker 03

  Wow.

• 16:04Noam Hassenfeld?

  And there is actually an international agreement that prohibits broadcasting on this frequency because it might interfere with signals coming from space.

• 16:13Speaker 03

  Hmm.

• 16:14Noam Hassenfeld?

  All of which is why the guy who found this signal a couple of years ago, he said, quote, "I'm convinced that the WOW signal certainly has the potential of being the first signal from extraterrestrial intelligence."

• 16:26Noam Hassenfeld?

  Now, he might be biased.

• 16:28Speaker 03

  Yeah, we all want to be that guy.

• 16:30Noam Hassenfeld?

  We all want to be the guy who discovers aliens, right? So I called up a different astronomer, just to get her sense of things. She's a radio astronomer, so she also kind of specifically looks for radio waves coming from space. She works at the University of Oregon. Her name is Yvette Sendis. And she's not exactly convinced by the aliens theory. I don't know if that's a shocker, but...

• 16:53Speaker 03

  I mean, I'm of the opinion that like, if you think we're alone out here, you're delusional.

• 16:59Noam Hassenfeld?

  I agree. But Yvette doesn't think this is the evidence that we're not alone.

• 17:04Speaker 03

  Fair.

• 17:05Noam Hassenfeld?

  And she told me there's this theory that maybe a blast of radiation could have hit a hydrogen cloud in deep space, which would explain why this signal was just on the one hydrogen frequency.

• 17:20Noam Hassenfeld?

  But no one's ever observed a radiation blast big enough to make the wow signal. So Yvette isn't into that either. She basically told me that it's easy to get excited about weird signals in radio astronomy. But given how many radio signals we're just always emitting all the time. If you detect something weird, odds are it's from us, like maybe a glitch on our end or something. And that's basically what she thinks might explain this. She thinks it might have been a satellite or interference or something. But even that explanation isn't perfect because the wow signal wasn't moving

• 18:03Noam Hassenfeld?

  relative to the stars in the background like a satellite would. It seemed like it came from somewhere really far away, which means that after all of this, we're left with a whole series of imperfect explanations. Unless it was the aliens. And aliens, if you're listening, maybe reach back out and let us know that you're listening a little more clearly than just-

• 18:28Speaker 01

  Who dis? New phone?

• 18:29Noam Hassenfeld?

  Yeah, maybe be a little more explicit than hydrogen. I don't know.

• 18:36Noam Hassenfeld?

  So that's where we are. What do you, what do you think? Do you think we figured out the wow signal?

• 18:39Speaker 03

  I don't know. I'm on the fence about this one.

• 18:42Noam Hassenfeld?

  Sounds like you're on the fence about a lot of them.

• 18:44Speaker 03

  Yeah.

• 18:46Noam Hassenfeld?

  Perfect.

• 18:46Speaker 03

  I knew I was gonna be bad at this game.

• 18:51Noam Hassenfeld?

  So you've got the mystery of birds' weird deep ocean holes. You've got Meredith's Roman concrete being way more durable than we would otherwise expect. And we've got what potentially caused the wow signal. They're all unexplainable. Or at least they all were unexplainable at one point. One of these potential mysteries has recently been figured out. And you'll have a chance to guess after the break.

• 19:21Speaker 01

  Boo boo boo.

• 19:24Noam Hassenfeld?

  Support for the show comes from Sonos. Hey, Sonos, give me some music.

• 19:30Noam Hassenfeld?

  All right. One of life's great mysteries. You're listening to music in the living room. You gotta grab something from the kitchen. You don't want to take the speaker with you, but you also don't want to miss that beat drop. And this is where I introduce you to the magic of Sonos. Sonos sent me a couple of speakers to try out, so I've got to move two here in the living room. And then I walk down the hall. I get in here to the kitchen, and I got a brand new Sonos Play right here on the counter. I can open the fridge, grab a seltzer.

• 20:12Noam Hassenfeld?

  And yeah, I didn't miss a second. And I can make each speaker louder or softer in the Sonos app, or if I'm using AirPlay, so I can make it louder in the kitchen without blasting whoever is still in the living room. It's all just easy, and it sounds amazing. These speakers are crisp. They've got great bass. And I'm especially impressed with the new Sonos Play. It packs this truly awesome sound in a much smaller size. So I don't know, if I want to head outside for a minute. It's super easy and yeah, pretty nice out. Visit sonos.com and build your system.

• 20:56Noam Hassenfeld?

  Sonos, love the sound, feel the system.

• 21:04Speaker 00

  Support for the show comes from TikTok. Being curious is endlessly rewarding. There are few better feelings, in my opinion, than asking a question and then having that question lead you down a rabbit hole. Might even be the beginning of a new set of interests that was not on your radar beforehand. Maybe you're looking up at the night sky one night and you start to wonder what the most distant thing you can see with a naked eye is. One place to start searching for answers to a question like that is TikTok. First, you might discover a video about the Andromedic Galaxy. And then, next thing you know, you're learning how to find different

• 21:45Speaker 00

  stars, or how to identify constellations, or how different astronomical discoveries got made, or how constellations got their names. You don't need serious equipment to make serious discoveries. Start with a simple question, and suddenly, you're somewhere you never expected to be. Your next discovery is waiting on TikTok. Learn more at tiktok.com/discovermore. At tiktok.com/discovermore.

• 22:19Speaker 04

  Support for the show comes from Engine. Running a small business means every dollar has to work hard. But if your team is still booking travel the old way, it's costing you more than you think. Engine is the fastest growing travel and spend platform in the country, built specifically for businesses like yours. Book a trip in as little as two and a half minutes, earn up to 10% back on hotels. And in 2025, Engine customers save more than $300 million on travel with zero booking fees, no contracts, and no BS. More than 1,000 businesses join Engine every month. Join them and get $500 when your business signs up and starts traveling at engine.com/box.

• 23:06Speaker 05

  It's nice to meet you. I'm Julia Kulia.

• 23:10Noam Hassenfeld?

  It's unexplainable or not, Julia, welcome back.

• 23:14Speaker 03

  Happy to be here.

• 23:15Noam Hassenfeld?

  We've got three mysteries to choose from, and only one of them has been explained.

• 23:23Noam Hassenfeld?

  Mystery one, what are all these weird holes at the bottom of the ocean? Mystery two, why was Roman concrete so strong? And mystery three, what's behind the wow signal? So just kind of tell me what you're thinking.

• 23:41Speaker 03

  Okay.

• 23:41Noam Hassenfeld?

  Maybe all of your anxieties.

• 23:44Speaker 03

  I'm very anxious right now. Just a baseline assumption I have is that the ocean is such a mystery to us. So I have a feeling that maybe we don't have an explanation for that one. And then as far as the concrete, it does feel like one that we would most feasibly have an answer to. But it's the kind of thing we would like just postpone figuring out because like big concrete, you know? Big concrete doesn't want better concrete? Because then we wouldn't have to fix the concrete all the time. Like, that's, you know, that's big money. It's all an elaborate ruse. It's a concrete conspiracy. It goes all the way up to the top.

• 24:28Speaker 03

  And yeah, the last one, I mean, I'm puzzled by, but I feel like maybe it has a dumb answer. Like, Bird really like, I don't know if you were trying to sway me or to bamboozle me with that. Maybe a bird went on there. I am a bamboozler, famously.

• 24:44Noam Hassenfeld?

  That's true.

• 24:46Speaker 03

  I had that feeling. I'm gonna guess and probably be wrong that the last one, the wow, is the one that we know. Wow. Wow.

• 24:57Noam Hassenfeld?

  Wow signal, final answer?

• 24:59Speaker 03

  Wow signal, final answer.

• 25:01Noam Hassenfeld?

  Okay, locking it in. Here's your answer.

• 25:06Speaker 06

  Instead of decorating, it gets better. That's what is happening with Roman concrete.

• 25:15Speaker 03

  They cracked open big concrete. They found the answer.

• 25:18Noam Hassenfeld?

  Big concrete's got to worry.

• 25:20Speaker 03

  Now who's shaking in their boots? Julia, your face two seconds into that quote.

• 25:25Speaker 01

  It just fell.

• 25:26Noam Hassenfeld?

  I know, you were just like, no.

• 25:29Speaker 01

  God damn it.

• 25:30Noam Hassenfeld?

  So, Julia, why Roman concrete is so durable has been figured out. And Meredith is going to tell you how this mystery was solved shockingly recently.

• 25:43Speaker 01

  Yeah, so it turns out that I, Meredith Hoddinott, am the bamboozler. Bamboozled.

• 25:50Speaker 03

  It's always the ones you don't expect.

• 25:51Speaker 01

  I know, underneath this smiley facade, it's just bamboozled down to the core. So we just heard from the MIT engineering professor I talked to, Admir Mashich. And he just loves concrete.

• 26:07Speaker 06

  The beauty of concrete is the fact that you have this incredible freedom of forms. You pour concrete and then it solidifies into a rock.

• 26:17Speaker 01

  He found out a couple of years ago why Roman concrete lasts thousands of years. And he says it all has to do with the mix. So, little concrete 101. Concrete is a bunch of sand and gravel and rocks, all held together by a glue made from processed limestone and clay called cement.

• 26:38Noam Hassenfeld?

  Heard of it.

• 26:38Speaker 01

  And throughout his career, Admir has looked at samples of Roman concrete from across the Mediterranean. Some samples even like preserved at Pompeii. And everywhere he looked, there were these little millimeter scale nuggets of mostly calcium.

• 26:55Speaker 06

  Yeah, that's a great, I never used the little nugget term, but I think it describes it incredibly well. It's gold for us in the sense that these little nuggets are super calcium rich.

• 27:10Speaker 03

  It's like a, like a Dairy Queen Blizzard that's not fully blizzed. It's like, got the big pieces.

• 27:16Speaker 01

  Yeah, and Admir says it was generally thought that these nuggets happened because the Romans were just not very good at mixing their concrete.

• 27:26Speaker 06

  The nuggets were considered bad material processing.

• 27:30Speaker 01

  But were all the Romans bad at mixing all of their concrete? Like, these nuggets are everywhere.

• 27:39Speaker 06

  You put so much effort in this material design, and then you don't mix it properly every single time. It's like, guys.

• 27:49Speaker 01

  And that gave Admire and his team an idea.

• 27:52Speaker 06

  I started to challenge this, that Romans consistently badly mix their concrete.

• 28:00Speaker 01

  So, cement, the glue of concrete, needs water to solidify. It's an important part of the chemistry that turns the calcium of limestone and other chemicals like silicates into such a powerful binder. But Admire says that he's been able to prove that the way the Romans mixed their cement meant that not all of the calcium combined with the water. These chemical reactions of mixing the concrete would get so hot that some of the water evaporated, leaving these calcium nuggets behind. Our modern concrete is much more uniform.

• 28:39Speaker 03

  It's a milkshake.

• 28:40Speaker 01

  Exactly.

• 28:41Speaker 06

  In the modern, you have all homogeneously distributed calcium, very boring, like, super mixed in. Whereas ancient Roman concrete is like, super heterogeneous.

• 28:52Speaker 01

  So, let's use our concretes to make some walls. Over time, tiny cracks start to form in the concrete, and water starts to leak in. In modern concrete, that's it. It's the beginning of the end. The water gets into the concrete in these cracks. It can freeze and thaw, making those cracks spread even further. The water could get all the way down to the, like, metal rebar that is like the structural support of the concrete structures. And then it starts eroding and rusting and beginning the whole crumbling process. But in ancient Roman concrete...

• 29:34Speaker 01

  That water leaks in, it hits one of these tiny calcium nuggets. That nugget dissolves, then recrystallizes and seals up the crack.

• 29:43Speaker 06

  So what makes Roman concrete magical is the chemistry behind healing process that's not taking place in modern concrete.

• 29:56Noam Hassenfeld?

  Fascinating.

• 29:57Speaker 01

  Roman concrete is literally healing itself. And Admire and his team aren't suggesting we make concrete the way Romans used to. There have been like a few engineering advances in the last couple of thousand years that are worth hanging on to. But this discovery is inspiring ideas for additives to help modern concrete heal itself.

• 30:20Noam Hassenfeld?

  Wait, so why wouldn't we just make concrete the way the Romans used to? Like if their concrete lasted for 2,000 years and our concrete lasts for like 100 years, why wouldn't we just? I also am confused by this. Yeah, that would make all the stuff with Roman concrete.

• 30:36Speaker 01

  Yeah. Right, so the Romans, they knew a lot of things, but they did not know how to make a concrete wall that was not like gigantically thick.

• 30:47Speaker 06

  In ancient Roman structures, there is no reinforcement, no steel. And so they build the structures with huge walls, thick walls, and everything was very, very bulky.

• 31:00Speaker 01

  And then while they could make these beautiful domes, it's actually really hard to make flat concrete surfaces without that structural support. So the top of the pantheon is just like this open skylight. And Admire was saying like, yeah, that's probably because they kept falling down because they couldn't figure out how to structurally support it.

• 31:22Speaker 06

  Maybe they even tried to make it and were not successful and then just left it open. I don't know. I'm, I should actually look into that.

• 31:31Speaker 01

  So if we want to keep our like thin walled elegant concrete skyscrapers with all those like real nice flat floors, we got to figure out how to take lessons from the Romans, but not do as they do.

• 31:48Noam Hassenfeld?

  So Julia, even though you didn't get the answer right here, you were you were stumped by by big concrete. We do have a consolation prize for you, which is a song I wrote.

• 31:59Speaker 03

  Incredible.

• 32:06Noam Hassenfeld?

  The Roman Coliseum somehow hasn't collapsed, even though they built it thousands of years in the past.

• 32:14Speaker 03

  Wow.

• 32:15Noam Hassenfeld?

  The Roman Hades got a dome and it's not reinforced. If we had tried that, it would fall right down to the floor.

• 32:24Noam Hassenfeld?

  There's still aqueducts, there's still seawalls, there's still marketplaces and that's. But finally scientists figured out why. It's because the Romans built them all with concrete. And tiny little hearts mixed clumps. They built them with concrete. These nuggets that were mostly made of calcium. Built them with concrete. Wasn't like they sucked at mixing. Built them with concrete. It's more like they just crush and build. Built them with concrete.

• 32:55Noam Hassenfeld?

  All roads lead to Rome and some of them well erode. But you know what they say about the ones that don't? They were probably built with clumpy Roman concrete. Tiny little hearts mixed clumps. They built them with concrete. These nuggets that were mostly made of calcium. Built them with concrete. Wasn't like they sucked at mixing. Built them with concrete. It's more like they just crush and build. Built them with concrete. And the nuggets, and the nuggets, and the nuggets.

• 33:28Speaker 03

  The nuggets. Nugget really is an excellent word.

• 33:35Speaker 03

  What a dream.

• 33:36Noam Hassenfeld?

  One last thing before we go. Julia, the Unexplainable series you've been working on, Good Robot, it comes out next week.

• 33:43Speaker 03

  Yeah, it does.

• 33:44Noam Hassenfeld?

  So I wonder if you could give us a little preview?

• 33:47Speaker 03

  I actually have a trailer for you.

• 33:49Noam Hassenfeld?

  Ooh, okay. Let's uh, let's listen.

• 33:55Noam Hassenfeld?

  Suppose in the

• 33:56Speaker 07

  Future, there's an artificial intelligence.

• 33:59Speaker 03

  I've been asking some very smart people a question that's been on a lot of our minds. Should we be worried about artificial intelligence? But the answers I got from the greatest minds in AI surprised me. One guy told a parable of an AI that could cause an apocalypse.

• 34:18Speaker 07

  Let's give this super intelligent AI a simple goal: produce paperclips.

• 34:23Speaker 03

  Via paperclip? Another woman cast AI as an octopus.

• 34:28Speaker 04

  We posit this octopus to be mischievous as well.

• 34:32Speaker 03

  And yet another story sounded like it was out of the Bible. She seems likely to drown. What should you do? Imagining AI as a savior.

• 34:41Speaker 00

  Like a god, kind of? Yeah, like a god.

• 34:43Speaker 03

  And all of these fantastical tales from the greatest minds in AI made me wonder, maybe even these people don't know what to think. I'm Julia Longoria. Good Robot, a series about AI, coming March 12th on Unexplainable, wherever you get podcasts.

• 35:05Noam Hassenfeld?

  Oh, man, I'm so excited for this, Julia. And we're so glad to have you on the team.

• 35:09Speaker 03

  Oh, thanks.

• 35:13Noam Hassenfeld?

  That's it for Unexplainable or Not. Thank you to our presenters, Byrd Pinkerton.

• 35:18Speaker 00

  That's me.

• 35:19Speaker 03

  The Bamboozler.

• 35:20Noam Hassenfeld?

  And Meredith Hadnot.

• 35:21Speaker 01

  That's my name.

• 35:23Noam Hassenfeld?

  Both Bamboozlers.

• 35:24Speaker 01

  You said it.

• 35:25Noam Hassenfeld?

  And thank you to our audience for joining us. If you have a mystery or a solved mystery you want us to tell on an upcoming game show, let us know. We read every single email. That's it for Unexplainable or Not. Goodbye.

• 35:46Noam Hassenfeld?

  This episode was reported by Byrd Pinkerton, Meredith Hadnot, and me, Noam Hassenfeld. Production and music from me, editing from Meredith, who also runs the show. Production support from Thomas Lu, mixing and sound design from Christian Ayala, and fact checking from Melissa Hirsch and Anuk Dusso. Thanks so much to Julia Longoria for playing our game this week. Her Unexplainable series Good Robot launches next week. And after that, you'll be hearing a lot more from her going forward. So get excited. Thanks to Lauren Katz for her help with this episode and honestly, all the game shows we've done so far. Thanks to Brian Resnick for co-creating the show.

• 36:26Noam Hassenfeld?

  And special thanks to both Anuk and Rob Byers, who are moving on to new projects. Anuk has been an incredibly thoughtful researcher and fact checker for us. And Rob has taught us all more about sound and engineering and just listening than I ever knew was possible. If you have thoughts about the show, send us an email. We're at [email protected]. And you can leave us a review or a rating wherever you listen. It really helps us find new listeners. You can also support this show and all of Vox's journalism by joining our membership program today. You can go to vox.com/members to sign up. Unexplainable is part of the Vox Media Podcast Network, and we'll be back with the first episode of our series Good Robot next week.

• 37:11Noam Hassenfeld?

  Next week.

• 37:40Speaker 04

  Support for the show comes from Engine. Running a small business means every dollar has to work hard. But if your team is still booking travel the old way, it's costing you more than you think. Engine is the fastest growing travel and spend platform in the country, built specifically for businesses like yours. Book a trip in as little as two and a half minutes, earn up to 10% back on hotels. And in 2025, Engine customers save more than $300 million on travel with zero booking fees, no contracts, and no BS. More than 1,000 businesses join Engine every month. Join them and get $500 when your business signs up and starts traveling at engine.com/box.

• 38:24Speaker 03

  Support for this show comes from Midi Health. Restless nights, persistent headaches, and feeling exhausted no matter how much you rest. For many women, these symptoms aren't separate struggles. They're connected to the same hormonal changes during perimenopause and menopause. But just because they're common doesn't mean you have to live with them. Midi can help you understand what's happening in your body and find the care and relief you deserve. Your symptoms have answers. Visit joinmidi.com right now to book your first visit today. That's joinmidi.com to book your first visit. Joinmidi.com. Insurance coverage varies. Check with your plan for coverage.